Yeah, that's my bad, I moved the instance URL to be similar to other apps and setup a redirect from admin.fedoraproject.org/mirrormanager to mirrormanager.fedoraproject.org, but forgot about the XML-RPC endpoint that only listens to POST queries, and those don't follow redirects.
The solution is to edit /etc/mirrormanager-client/report_mirror.conf and change the server= line at the beginning to: server=https://mirrormanager.fedoraproject.org/xmlrpc
I'll update the RPM as soon as possible, but the config file will not be updated by the rpm update alone as you've most likely changed it from its default state by adding your mirror definition.
Sorry for the trouble!
There's been a very big mirrormanager update, if you're experiencing other troubles please report them here or on https://pagure.io/fedora-infrastructure/ and I'll have a look at them asap.
Thanks